1. Field of the Invention
This invention pertains generally to dynamoelectric machines, and more particularly to a method and apparatus for removing a resin impregnated coil from a core slot of such a machine.
2. Related Art
A process for globally vacuum impregnating entire large electric generated stator cores was developed by the Siemens Corporation in Germany approximately ten years ago. The process involves a large vacuum tank capable of holding the entire wound stator core as well as space for power rollers to rotate the core while it is partially submerged in liquid resin in the vacuum tank. A significant number of large electric generators have been manufactured using this process since it was introduced. One of the advantages of this process is that all the coils and bracing is installed “dry” so the handling of wet and messy epoxies is not required, which saves a considerable amount of time. Secondly, the liquid resin under the driving force of the vacuum penetrates voids between the coils, stator laminations, and bracing components, which improves both heat transfer and mechanical strength. Thirdly, elimination of gaps between the sides of the stator coils and the stator iron should also eliminate any partial discharge which has been found to occur in some conventionally wound machines having higher operating voltages. One disadvantage associated with the global vacuum impregnated process is the large extraction force required to remove a failed stator half coil. The prior art method of removing such a half coil includes drilling and tapping dozens of holes into each stator half coil, installing steel all-thread rods in each drilled and tapped hole along with a hydraulic cylinder for each all-thread rod. All the cylinders are connected to a common manifold so that all cylinders act in unison in order to extract the stator coil from the slot. Special tooling jigs are required to perform the drilling and tapping operation in order to avoid stator iron damage, such as what would happen if the drill were to penetrate too deeply or wander. In addition, copper is also somewhat difficult to machine due to its tendency to plug up the cutting tools. Tapping copper is very difficult, and adequate lubrication on the tap must be used and the chips must be removed from the tap. This is done to avoid gumming up the tap to the point it becomes stuck in the copper, which can result in a broken tap.
Accordingly, a new method is desired for removing globally vacuum impregnated coils from the core slots of a dynamoelectric machine that is less time consuming, simpler to implement and has less potential for stator iron damage.